In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each transmitter. When an individual subscriber or other authorized user of a particular cellular telephone wishes to place a phone call, he dials in a telephone number and presses the "Send"button. In response, his cellular telephone transmits its ESN and MIN to the cellular network so that the individual subscriber can be charged for the telephone call.
Cellular telephones provide a great advantage over fixed telephones because the user has almost unlimited mobility. A user subscribes to a cellular service in one location, such as his home or office, but can use the cellular telephone in other locations as well. If the user operates the cellular telephone to place or receive call outside the area of coverage of the cellular telephone network to which he has subscribed, the cellular telephone operates in a "Roam" mode. Cellular telephone networks are in place in most areas of the United States and in many areas throughout the world. The user can operate the cellular telephone in the Roam mode to place a telephone call from virtually any location in the world that has cellular network coverage. Similarly, the user can receive a call from a fixed telephone location or any other cellular telephone whether the user is in the area of coverage of the cellular telephone network to which he has subscribed or is roaming in another area. This form of communication thus offers a great deal of freedom for the user and those communicating with the user.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by counterfeiting the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular network without paying for the service. The ESN and MIN of a cellular telephone can be obtained by a counterfeiter electronically monitoring the initial transmission of the user's telephone, and then programming the detected ESN and MIN into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN is by itself inadequate to protect a cellular telephone system from misuse by counterfeiters.
While the cellular telephone is identified by its ESN and MIN, it cannot be considered as an authorized cellular telephone because it is not known whether the ESN and MIN have been transmitted by the authorized cellular telephone or a fraudulent cellular telephone. For purposes of the present description, a cellular telephone identified on the basis of the transmitted ESN and MIN is designated as an unauthenticated cellular telephone until it is determined to be an authorized cellular telephone or a fraudulent cellular telephone. The designation of the unauthenticated cellular telephone as authorized or fraudulent must be made on the basis of some form of analysis beyond a simple check of the ESN and MIN to see that these are valid numbers.
In an effort to provide additional security, some cellular systems and other wireless services, authenticate mobile units based on the radio frequency (RF) transmission of data by the mobile unit during a call set-up process. Rather than identify the mobile unit by its ESN and MIN alone, the system identifies a cellular telephone by its transmission characteristics. In this manner, the cellular system operator can reject calls from illegitimate cellular telephones even when those cellular telephones transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient response is used to identify the type of transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other transmission characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910 issued to Rudokas et al. on May 30, 1995 ("the Rudokas patent"), describes an identifier, such as a radio frequency signature, that can be used to positively identify an authorized cellular telephone or a fraudulent cellular telephone. Other types of signature authentication systems are also known in the art and need not be described herein. These transmission characteristics, from whatever source they are derived, can be processed in different manners to create a "fingerprint" of the individual transmitter. The analogy with fingerprints is used because each transmitter fingerprint is believed to be completely unique. The transmitter fingerprint can be used to determine whether the transmission characteristics of the unauthenticated transmitter match the stored fingerprint of the authorized transmitter corresponding to the transmitted ESN and MIN. In such manner, the fingerprint is used with cellular telephone calls to authenticate a cellular telephone.
For purposes of the present description, a present call being analyzed is designated as a valid cellular telephone call from an authorized cellular telephone or an invalid call from a fraudulent cellular telephone. The process of comparing the transmission characteristic of the unauthenticated transmitter from the present call with the stored fingerprint must be performed in real-time to effectively deny or terminate invalid calls. Unfortunately, no fingerprint analysis system is 100% effective. The transmission characteristic rarely, if ever, precisely matches the stored fingerprint. Thus, it is possible that a fraudulent cellular telephone will be identified as an authorized cellular telephone. Conversely, it is possible that an authorized cellular telephone will be identified as fraudulent. While the former misidentification may result in incorrect charges to the customer, and the resulting loss of revenues to the service provider, the latter misidentification results in great inconvenience to the customer and the disruption of valid calls due to the misidentification of an authorized cellular telephone as fraudulent.
Some existing cellular telephone systems use other analysis techniques to classify cellular telephones as authorized or fraudulent. Many of these analysis techniques cannot be performed in real-time. For example, some cellular telephone service providers identify certain destination telephone numbers as "known fraudulent numbers" and will not permit cellular telephone calls to be placed to those numbers. However, the use of a known fraudulent numbers list will not aid in terminating the first fraudulent cellular telephone call to that number. A particular cellular telephone call can subsequently be designated as invalid based on factors such as customer complaints about the bill. In this case, the destination telephone number called by the fraudulent cellular telephone is added to the list of known fraudulent numbers. The cellular telephone system checks the list of known fraudulent numbers to prevent any subsequent telephone calls from being placed to a known fraudulent number.
Some cellular telephone service providers are forced to adopt extreme measures to reduce the illegal use of cellular telephones. For example, in geographic areas known to be inundated with illegal users, some cellular telephone service providers will not allow cellular telephones to operate in the Roam mode. Unfortunately, this form of fraud prevention comes at the cost of great inconvenience to the legitimate user who may not be able to enjoy the full freedom of mobility offered by cellular telephone technology.
Each type of detection technique described above offers some degree of protection against fraudulent cellular telephone use. However, each detection technique suffers from disadvantages in either it ability to detect fraudulent use, the processing time required to detect fraudulent use, or in the burden imposed on cellular telephone customers by limiting their ability to use their cellular telephone or by making it less convenient to do so.
Therefore, it can be appreciated that there is a significant need for a system and method for detection of wireless telephone fraud and that has the advantages of different fraud detection techniques, but minimizes the inconvenience to the customer. The present invention provides this and other advantages as will be illustrated by the following description and accompanying figures.